Pelleting was introduced into the United States feed industry in the mid-1920's to improve feed utilization, increase the density of the feed and improve handling characteristics. Prior to about 1930 several different types of pelleting machines were utilized. Toward the end of the 1920's the flat die pelleting machine and ring die pellet mill were initially developed. While the flat die machine is still in use in certain applications, the ring die pellet mill quickly became the preferred design and was quickly adopted by the animal feed industry and remains the form of pelleting machine of choice today. In addition to the ring die pellet mill itself, auxiliary equipment was developed including conditioners, cooler/dryers, and related process equipment.
The early pelleting process involved mixing the feed ingredients and pelleting them with no further treatment, with the rationale being to prevent alterations to vitamins and proteins by avoiding the addition of heat to the feed. In the late 1930's, however, some processors began subjecting pellet-forming mixtures of animal feed to water and steam by passing the mixtures through a conditioner prior to introduction into the pellet extruders. The addition of steam improved production rates, reduced die wear, and improved pellet quality. Subsequently, the industry quickly adopted steam conditioning during pellet formation and this has remained an integral part of the pelleting process to date.
During steam conditioning, live steam is injected into the feed mash as it is conveyed through the conditioner which generally consists of a cylindrical tube with a rotating shaft upon which numerous paddles or picks are mounted. Meal conditioning with steam is a prerequisite for the compression of the meal or mash into pellets. Heat and water from the steam serve to activate binders in the meal particles, e.g., protein and carbohydrates, soften them and bring cohesive properties onto the surfaces of the particles. When the mash is compressed through a die, the particles are compacted and stuck together to form pellets. The condensing steam, of course, increases the temperature and moisture content of the mash. Moreover, since the steam is injected directly into the feed mash, the chemicals used to treat the boiler must be FDA approved.
The focus on research into the pelleting process since the 1960's has been on improving the steam conditioning operation, with emphasis on increasing the retention time and increasing the temperature to which the mash is conditioned. One of the more recent developments was a pressure pelleting system in which the conditioner and pelleting die cavities were pressurized, to provide for use of higher temperatures and longer conditioning times to improve pellet durability and increase the production rate. The use of increased temperatures and conditioning times, however, runs counter to the inclusion of virtually any heat sensitive or labile ingredients which may also desirable in complete animal feeds.
What is needed in the art, therefore, is a method of producing animal feed in the absence of an intentionally added heat source, such as steam, such that heat labile additives can be added to the feed. Moreover, methods and machinery that do not utilize steam injection may be more economical to operate since the methods would not require a boiler for steam production. The lack of a boiler would, in turn, remove the necessity of seeking FDA approval of certain chemical treatments of the machinery.